(1) Field of the Invention
This invention relates to a thin type sealed cell comprising a cell case or enclosing an electrolyte and a power generating element, the cell case made of a sheet type laminated material having a sectional surface in which each of the layers composing the laminated material is exposed, the layers being an aluminum layer and an adhesive layer and a resin layer. The invention more particularly relates to a construction of a cell case using the laminated material and a method of producing such a cell case.
(2) Description of the Prior Art
In recent years, more reduced sizes and weights have been required for electric cells in connection with a size reduction trend of electronic devices. In view of such requirements, the present inventors have suggested in Japanese Unexamined Patent Publication No. 10-214606 a thin sealed battery utilizing a cell case made of a laminated material formed into a bag-like shape in which an aluminum layer is sandwiched by resin layers formed on both surfaces of the aluminum layer with an adhesive layer interposed therebetween. According to the construction suggested in the above Japanese Unexamined Patent Publication No. 10-214606, owing to the fact that such laminated materials are lightweight and thin, and moreover can be hermetically sealed easily by heat-bonding, a small-sized and lightweight thin sealed battery capable of achieving very high productivity is realized.
While the above prior art thin sealed battery has such a desirable feature as described above, it also has certain drawbacks with regard to a storage life and reliability of the cell. More specifically, a sheet type laminated material used for such cells has exposed sectional surfaces in which an aluminum layer, an adhesive layer, and a resin layer are exposed, and therefore if the exposed sectional surfaces exist inside an enclosure chamber, which encloses a power generating element and an electrolyte, the electrolyte infiltrates from the exposed sectional surfaces. As a result of this, an adhesive strength of the adhesive layers is weakened, and the aluminum layer and the resin layers are detached in the end, resulting in a leakage of the electrolyte out of the cell.
If such detachment of the aluminum layer and the resin layer occurs, not only is the leakage of the electrolyte caused, but also an infiltration of moisture into the cell is induced. As a result of the infiltration of water, for example, in batteries using a non-aqueous electrolyte including LiPF.sub.6 and the like as a solute, a fluoric acid is generated by the reaction of the water and LiPF.sub.6, and the generated fluoric acid further reacts with the aluminum. As a consequence of this, the aluminum layer is corroded, and a further leakage of the electrolyte is caused from the corroded part. These result in a rapid deterioration of cell performance.
In addition, if the detachment of the aluminum layer and the resin layer is caused by the reaction of the adhesive and the electrolyte in the exposed sectional surfaces, the aluminum layer thus exposed makes contact with a negative electrode, resulting in an internal short circuit. Again, this results in a rapid deterioration of cell performance.